Google Scholar is the search company Google's engine for searching scholarly or academic materials. Google Scholar searches many publisher's journals, academic websites, institutional repositories, and other similar sources, including the ScienceDirect and JSTOR databases. In Google Scholar you can find:
Google Scholar is strongest in science, technology, and medicine.
Like regular Google, Google Scholar using a "relevance" algorithm to determine the ranking of the results. This takes into account the position of your search words in the titles, abstracts, and other descriptive material about the document.
The "Scholar" in Google Scholar is not the equivalent of the "Scholarly Journals" or "Peer-Reviewed Journals" check boxes in the databases. Scholarly in this case refers to non-commercial sites, like universities, and to publisher websites. Google Scholar does have scholarly and peer-reviewed articles, but also has non-reviewed articles and presentations, pre-prints (pre-publication versions of articles), reports, and a wide variety of other materials that exist on scholarly websites or have been cited in other scholarly works.